1. Technical Field
This invention relates in general to audio amplifiers and, more particularly, to a track-and-hold circuit to reduce noise in a headset amplifier.
2. Description of the Related Art
Audio circuitry often suffers from annoying “pop” noises as power is applied to or removed from the audio amplification circuitry. While the popping can be annoying in any application, it is particularly annoying in headset applications, where the speaker is firing directly into the user's ear.
Headsets are becoming more and more popular with mobile processing devices and especially in communications applications, such as with mobile telephones (including devices such as personal digital assistants that may be used as a telephone, either through cellular circuitry or as a voice-over Internet-protocol, or VOIP, device). In conjunction with mobile processing devices, power consumption is an especially important consideration, and power may be frequently switched on and off to the headset amplifier.
FIG. 1a illustrates a general diagram of a receiving (RX) voice channel in a headset amplifier. A DSP (digital signal processor) 10 generates the voice data in digital form. A digital filter 12 receives the digital voice data and passes the filtered data to a DAC (digital-to-analog converter) 14. The output of the DAC 14 (shown here as a differential voltage) is applied to the input of a headset amplifier 16. The output of the headset amplifier 16 is coupled to a speaker 18 through an AC coupling capacitor 20.
FIG. 1b illustrates the state of the art in filtering using an external filtering capacitor 22 to reduce popping noises. In this embodiment, when the headset amplifier transitions from a power-off state to a power-on state, the VMID amplifier 24 (which provides a voltage reference equal to the power supply divided by 2) is powered-on and the headset amplifier output is shorted to VMID using the CHG switch. The settling time of the output OUT is equal to the settling time of VMID, which is dependant on the settling time of the RC-filter (R1C1). The CHG switch is disabled, and the headset amplifier is then turned on.
For the power-off phase, the headset amplifier is turned off and the DCHG switch shorts the output pin of the output amplifier (OUT) to ground. In this case, the settling time depends on the settling time of the AC-coupling capacitance C2.
This solution has several problems. While the DSP 10, digital filter 12, DAC 14 and headset amplifier are generally formed on a single integrated circuit, the filtering capacitor 22 is an external component on the order of 4.7 μF, which is too large to place on the integrated circuit. Accordingly, a design such as that shown in FIG. 1b requires both an external capacitor and an extra pin from the integrated circuit. Second, the settling time during the power-up, which depends on the R1C1 filter settling time, can be excessive. Third, the discharge phase is lengthy and may not meet timing constraints in certain applications. The only way to decrease the power down time is to increase the size of DCHG switch (typically a MOS transistor) drastically.
While the cost of the capacitor 22 and extra pin is small for a single device, given the large number of devices generally produced, the overall cost can be significant to the manufacturer.
Accordingly, a need has arisen for a fully integrated solution to eliminate popping noises.